In it's defence [just a little ], FSX is doing a lot in terms of rendering - how many titles allow you to circumnavigate the entire globe without any noticeable loading, or allow you to to from sea level to the outer limits of the atmosphere, revealing a true globe. Pick a fast military jet and fly strait up and watch the familiar shapes of the continents map out below you - now that's draw distance.

Take a look a Phil Taylors blogs on the amount of calculations involved with high autogen density’s, it's staggering.

There is nothing else doing this stuff, and is one reason it has limited performance..it is also hard to beat visually, esp. in respect to what it's for (it can't be compared to a FPS) just look at some of the shots here.

But having said all that, I'm sure it could be better, as one of the above posters pointed out, there is a lot of legacy code in there, which has to bog things down, but hopefully that will change for the next version.